1. Field of the Invention
The present invention relates generally to activity monitoring of a digital signal to provide an alarm indication when the digital signal input being monitored becomes inactive. More specifically, the invention also relates to a precise resolution and signal loss threshold setting technique for digital activity monitoring that is substantially invariant with temperature or power supply voltage variations.
2. Description of the Prior Art
In the known prior art, a retriggerable monostable multivibrator has been widely used to detect the loss of activity of a digital signal. The basic pulse duration of the monostable multivibrator defines the period of signal inactivity, e.g. the signal loss threshold, before an alarm is raised. Such an alarm is used in a telephone central office to indicate loss of input signal. The monostable pulse width is programmed by selection of external resistance and capacitance values. When the input signal is active it triggers and retriggers the monostable multivibrator to extend the output pulse. Thus the output of the monostable multivibrator remains HIGH (logic "1" state) as long as the time between successive signal pulses is less than the basic pulse duration of the monostable multivibrator. As soon as the time between two signal pulses is greater than the basic pulse duration of the monostable the output returns to its stable (LOW or logic "0") state indicating a loss of the monitored signal and generating an alarm.
The aforementioned known technique has two main disadvantages. The first disadvantage is the variation of the basic pulse duration of the monostable, and hence of the signal loss threshold, with component value and with temperature and power supply voltage variations. The second disadvantage is that the prior art circuit does not lend itself to integration in state of the art digital Application Specific Integrated Circuit (ASIC) technology, due to its analog nature.
Additional examples of the known prior art are U.S. Pat. No. 4,455,456 which describes a digital supervisory circuit for a telephone system in which the decoding or extracting of two supervisory signals which are superimposed on an analog telephone signal is provided. U.S. Pat. No. 4,196,362 describes a circuit for the generation of a clear or reset pulse upon application of a power supply voltage to the circuit so that the rest of the circuits can be placed into a known state after the application of power to the circuits. U.S. Pat. No. 4,017,724 describes a circuit for measuring and indicating the state of charge of a battery in a battery operated system. German patent No. 28 04 950 describes a circuit for the generation of signals having a selectable repetition frequency.